RSS v Atom

Speaking as someone who is keen to understand but completely clueless, my basic reading has led me to understand there are two versions of RSS and then Atom on top, why do we have three seemingly incompatible 'standards'?

This is not an opinion; I have about 20 blogs in my newsreader. I started going through them to see which were Atom and which were RSS. Of the first 10 I looked at, 9 were Atom. The other one, I couldn't really tell. But on closer investigation (i.e. displaying the source), it's Atom as well. One could draw a conclusion from that small sample.